Well, in spite of being very fat, I've felt some of my babies (internally) very early, and this one I just felt from the outside (hand on my stomach) last night--at about 17 weeks.
A lot depends on your baby's position and where the placenta is, too.
Also, babies are all different. Some are very avoidant of anything pressing into their space (hands, doppler, ultrasound wand), and others are, for lack of a better word, "interested" and may kick directly at that spot. At about 20 weeks with my first child, dh had his arm draped over my belly as we were falling asleep, and I could feel that baby exploring all along the length of dh's arm, and then a very strong kick right to it. Dh was amazed. :)
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